Re-organise test files for easier navigation

[elichad]

The way our tests are organised isn't particularly consistent across our different apps and models. Sometimes view tests will be in test_views.py, sometimes they're under test_<model>.py, etc., and this is similar for forms, filters, and so on. This makes it a bit harder to determine if a feature is tested and where those tests live. Our class names are more consistent, but still not perfect.

It would be nice to standardise how we name test classes and files, and the types of test that should belong to particular files.